2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 13
Presentation Time: 4:45 PM


MUNDIL, Roland, Berkeley Geochronology Ctr, 2455 Ridge Rd, Berkeley, CA 94709-1211, RENNE, Paul R., Berkeley Geochronology Ctr/UC Berkeley, 2455 Ridge Road, Berkeley, CA 94709, ZHOU, Zhensheng, Earth and Planetary Sciences, UC Berkeley, Berkeley, CA 94720, METCALFE, Ian, Asia Centre, Univ of New England, Armidale, NSW 2351, Australia and FOSTER, Clinton, Petroleum & Marine Division, Geoscience Australia, Canberra, ACT 2601, Australia, rmundil@bgc.org

In order to understand the timing and potential causes of the most severe extinction event in the Phanerozoic it is essential to study and correlate both marine as well as terrestrial sedimentary records. New radio-isotopic age results from both U/Pb dating on zircons and 40Ar/39Ar dating on sanidines from volcanic ash falls constrain the age of the Permian-Triassic boundary in marine strata (as defined by the FAD of the conodont Hindeodus parvus) to an age of 253 Myr and the main pulse of the extinction to an age of 254 Myr (ages are based on the U/Pb decay constants). Whereas zircon populations from individual ash layers are complex and prohibit the extraction of a statistically robust age in many cases, the analysis of multiple ash layers in different stratigraphic levels by single-zircon techniques yield meaningful and reproducible ages throughout the final stage of the Permian. In addition, new 40Ar/39Ar age data confirm the U/Pb ages, with the 40Ar/39Ar ages being systematically younger by 1-2 % than U/Pb ages, but yielding identical intervals between dated ashes. This bias is most likely due to systematic errors in the current calibration of the 40Ar/39Ar system. It has been notoriously difficult to precisely correlate marine and terrestrial sedimentary records due to a lack of reliable radio-isotopic ages. New U/Pb age data on single zircons from a volcaniclastic layer conformably underlying the Rewan Fm. (Bowen Basin, Queensland, E Australia) yield a precise and robust age of 255.7 ± 0.6 Myr. The tuff shows evidence of soft sediment deformation by the quartzose sands of the Rewan Fm., suggesting that the units are contemporaneous. The tuff yielded few palynomorphs, but an assemblage 3.82 m above the contact within the Rewan Fm., belongs to the Lunatisporites pellucidus Zone and lacks Aratrisporites spp.. Traditionally the Rewan Fm. is regarded as a Triassic unit, as it post-dates the Glossopteris coal-bearing strata. However, our new age, in combination with our detailed age results from marine strata in S China, correlates this part of the succession with the latest Permian. This result facilitates improved understanding of the timing of the biotic crisis, but additional radio-isotopic ages, in combination with bio- and magnetostratigraphy, from terrestrial, paralic and marine sections are essential to reconstruct the events across the P-T transition.